1. Field of the Invention
The invention concerns an adhesive tape, the adhesive layer of which contains electrically conductive particles. The particles can afford electrical conductivity through the thickness of the adhesive layer while being laterally spaced so that the layer is electrically insulating in lateral directions; or the particles can form electrically conductive stripes that afford electrical conductivity both laterally and through the thickness of the adhesive layer.
2. Description of the Related Art
Modern electronic devices are becoming so small and delicate that it can be virtually impossible to interconnect electrodes of two such devices either mechanically or by soldering. It also may be necessary to dissipate heat and static electricity from such a device, e.g., from a semiconductor die or chip.
U.S. Pat. No. 4,113,981 (Fujita et al.) uses an adhesive layer for individually electrically interconnecting multiple pairs of arrayed electrodes. The adhesive layer includes spherical electrically conductive particles of slightly less thickness than the adhesive. When the adhesive layer is compressed between electrodes, individual particles bridge the electrodes. The particles are randomly distributed throughout the adhesive layer, but Fujita indicates that if the particles comprise less than 30% by volume of the layer, they will be laterally spaced so that the intervening adhesive will insulate against short circuiting between laterally adjacent electrodes.
In PCT Publ. No. WO 85/04980 dated Nov. 7, 1985 (Dery et al.), an adhesive layer is made electrically conductive by incorporating what Dery calls "conductive units" of electrically conductive particles. "Depending upon the size of the particles used, the conductive unit may be a single particle or a plurality of clustered particles" (sentence bridging pp. 8-9). "The particles have a tendency to form clusters during the mixing of the composition. These clusters are of sufficient size to permit conductivity through the layer of adhesive composition" (p. 10, lines 3-5).
Although not mentioned by Dery, the tendency of electrically conductive particles to cluster makes the adhesive layer laterally electrically conductive across the width of each cluster, so that adjacent electrodes could be short circuited by a cluster having greater breadth than the spacing between those electrodes.
Clustering can be minimized by employing electrically conductive particles that are substantially as large as the thickness of the adhesive layer, and locating the particles in preselected segments of the adhesive layer. Coassigned U.S. Pat. No. 4,606,962 (Reylek et al.) teaches, beginning at col. 2, line 39, three techniques for doing so. The electrically conductive particles of Reylek preferably are spherical, exceed the adhesive thickness, and are soft so that when the adhesive layer is compressed between two flat rigid plates, the particles are flattened, thus providing small, flat conductive areas at both surfaces of the adhesive layer. Instead, each of the particles may be an aggregate of tiny fused granules such as granules of a metal which is at least as deformable as substantially pure silver.
In coassigned EPO Pat. Appl. Publ. No. 0,330,452 dated Aug. 30, 1989 (Calhoun et al.), electrically conductive particles are individually transfered to the adhesive layer, but FIG. 4 shows that sometimes two or more particles are deposited together. The electrically conductive particles of Calhoun are equiax and have sufficient hardness to penetrate into electrodes to be connected. The use of the Calhoun adhesive layer to interconnect multiple pairs of arrayed electrodes would involve no danger of shortcircuiting provided that the total breadth of the largest number of particles that might be deposited at one position did not exceed the spacing between laterally adjacent electrodes.
Multiple electrodes to be interconnected typically are arrayed on a flat electrically insulative surface and protrude so slightly above that surface, e.g, from 15 to 150 .mu.m, that the electrode bearing surface can be considered to be substantially flat. When an adhesive layer containing laterally spaced electrically conductive particles is employed to interconnect two such arrays, the adhesive can be squeezed into lateral spaces between adjacent electrodes, especially when the adhesive is flowable at the bonding temperature. The result of doing so is illustrated in U.S. Pat. No. 4,680,226 (Takeda). See also U.S. Pats. No. 4,731,282 and No. 4,740,657 (both Tsukagoshi et al.) and U.S. Pat. No. 4,735,847 (Fujiwara et al.). Any such lateral flow of the particles into the lateral spaces between electrodes can increase the danger of short circuiting of closely spaced electrodes.
Another problem is that to use an adhesive layer to interconnect tiny electrodes, it must contain a sufficient concentration of the electrically conductive particles or clusters of particles to ensure that at least one particle or chain of particles makes good electrical contact between every pair of electrodes. Because electrically conductive particles interfere with adhesion, there is a hazard that the particles might interfere with the ability of the adhesive layer to maintain a permanent bond, especially in uses involving wide variations in temperature and humidity. In some uses, even an occasional failure can be disastrous.
Coassigned U.S. Pat. No. 4,548,862 (Hartman) also uses an adhesive layer for interconnecting multiple pairs of electrodes, doing so with electrically conductive particles that are smaller than the thickness of the adhesive layer. While forming the adhesive layer, a magnetic field aligns the particles into a large number of discrete, laterally spaced chains extending through the thickness of the adhesive layer.
In the adhesive layer of U.S. Pat. No. 4,448,837 (Ikeda et al.), similar electrically conductive chains are formed of relatively coarse particles, and finer electrically conductive particles are dispersed near at least one surface of the adhesive layer.
In coassigned U.S. Pat. No. 4,546,037 (King), an adhesive layer contains electrically conductive particles that have been magnetically aligned to form nonintersecting stripes, each containing a continuous array of contacting electrically conductive particles to make each stripe electrically conductive over both its length and thickness. Such an electrically conductive tape can make individual electrical connections between two banks of electrodes which cannot be superimposed.
3. Other Prior Art
In U.S. Pat. No. 4,008,300 (Ponn), a slurry of electrically conductive particles is forced under pressure into parallel perforations that extend through a thin elastomeric sheet, thus forming resilient electrically conductive rods which bulge out beyond the ends of the perforations. Upon being clamped between flat surfaces, lateral forcible expansion of the individual rods puts them under pressure, thus assuring that each rod electrically interconnects facing pairs of electrical terminals on the two flat surfaces, e.g., terminals on printed circuit boards. Ponn does not use an adhesive and instead depends upon a mechanical clamp to maintain the electrical connections.
An electrical connector similar to that of Ponn is described in U.S. Pat. No. 3,680,037 (Nellis et al.). The element of the Nellis Patent that is comparable to Ponn's elastomeric sheet is called a "retainer sheet" or a "dielectric retainer" that can be a themosetting or thermoplastic resin.